Image forming devices such as laser printers and photocopiers typically include at least one pulse width modulator (PWM) for generating video signals. A video signal comprises a succession of various width pulses derived from image data, such as a bitmap. The video signal is received by a print engine and controls the application of light or other type of beam to areas on a charged drum. The light discharges the areas to which it is applied. The drum is then coated with toner, which clings to the discharged areas. A media, such as a sheet of paper, is then exposed to the toner on the drum and the image is thereby formed on the media.
The quality of a printed image is highly dependent upon the accuracy of the PWM and its capacity to time the pulses with precision. Pulse timing is based upon the image data and dependent upon the frequency of the print engine, i.e., the pace at which the light beam moves along a scan line, which may change from pixel-to-pixel.
An important consideration of PWM design is the relationship between the PWM frequency and the print engine frequency. More and more frequently, print engines are designed to operate at frequencies that are not evenly related to the PWM clock. In addition, the frequency of the print engine may change pixel-to-pixel, further complicating the PWM design.
The testability of PWM circuits in imaging devices is complicated due to their high operational speed, which may reach into the GHz range. Also, PWMs utilize analog circuitry, in addition to digital circuitry, to achieve the goal of fine pulse placement at the frequency of the print engine. It is difficult to test analog circuits and/or high frequency circuits because such testing requires measurements at precise moments, oftentimes defined by only a few picoseconds.
Thus, it would be desirable to provide an improved digital PWM technique for use in an image forming device.